Short answer: when the Queen of England says so.
“Harry and Meghan”(TM), the Duke and Duchess of Sussex, with Harry sixth in line for the throne, have been told in no uncertain terms that they can’t go around selling stuff using “Sussex Royal” as a brand name (https://www.dailymail.co.uk/news/article-8030241/Prince-Harry-Meghan-Markle-stop-using-Sussex-Royal-brand-territory-spring.html). They bailed out on being working members of the royal family and went west to become celebrities.
They’re kind of cheesed off about not being allowed to use the royal family as a sales gimmick. “The queen doesn’t own the word ‘Royal,'” they proclaimed on their website. Even so, just to be nice guys, the Duke & Duchess will voluntarily stop using the “Sussex Royal” brand… which they had already mobilized to sell clothing, stationery, and bandanas.
You can see how this could get out of hand. Next thing you know, you’ve got Sussex Royal fish sticks, cat toys, video games, and borscht. “They simply cannot be allowed to market themselves as royals,” is the word from Buckingham Palace.
Although Harry and Meghan(TM) have left their duties as members of the British royal family, the UK and Canada will split the tab for their security–three to six million Pounds per year. Oh, well, it’s all public money… grows on trees, you know.
Just out of curiosity, what would happen if Harry & Meghan(TM) flatly refused to stop selling stuff as Sussex Royal? Ka-boom? Sleep with the fishes? They don’t seem to be in any hurry to find out.
Stay tuned for the Harry & Meghan(TM) hair-restoring tonic.